Lithium-ion (Li-ion) batteries are being produced in ever increasing numbers, particularly as the size of Li-ion batteries increases and the market for EV's, Backup Power and Consumer electronics keep growing. There is a strong need to be able to recycle these batteries after their useful life in that application has expired. One approach that has been proposed is described as “direct” recycling, in which instead of melting the batteries down to recover only the valuable metals from the cells such as Cobalt, the high value active anode and cathode materials are removed from the cell to be “refurbished” so that they can be reused in new Li-ion cells. The approach has the advantage of achieving potentially major reductions in the cost of Li-ion cells since the active materials typically are one of the most expensive components of the cells, and of being the most environmentally friendly approach, requiring very little energy use relative to other forms of recycling. Furthermore, the approach has the potential to be self sustaining, as the high value of the re-sold recovered material would fully cover the costs of recycling the material, independent of the metals contained in them and their current market value.
A key step in the process of recycling Li-ion batteries using a direct recycling approach is the separation of the active material. powders (such as LiCoO2 or graphite) from the rest of the cell mass and from each other. The typical process begins with the cell going through a hammer mill in which the entire cell and its components are essentially reduced to a fine powder referred to as the “black mass”. Along with the anode and cathode active material, the “black mass” may contain copper powder and aluminum powder from the current collector, steel particles from the cell casing, polymeric binder, polymeric film pieces from the separator, and various forms of high surface area carbon. Much of these components can be removed by sieving the black mass or magnetic adsorption. However, because of the similarity of the cathode active material (metal oxide) and anode active material (graphite) in terms of morphology, it is often very difficult to separate the two from each other. Unfortunately, it is critical that they be separated and purified since any cross contamination can have negative effects on the performance and specifications of the two active materials, Various methods have been used to separate these materials. RSR Technologies has proposed using conventional flotation separation processes, commonly used in the mining industry, to separate the two components. The process typically involves mixing the mixed powder into water. While all of the black mass material will have a tendency to settle to the bottom of the vessel because of the relative densities of the components (ex. ˜2.2 g/cc for graphite and ˜4.4 g/cc for LiCoO2), in flotation separation the materials are selectively functionalized such that the particle surfaces are either hydrophobic or hydrophilic. Bubbles are created at the bottom of the typically aqueous separation chamber. As the bubbles rise, the hydrophobic material spends more time associated with the rising non-aqueous bubbles, while the hydrophilic material spends more time in the aqueous phase. The result is that the hydrophobic phase is primarily carried to the top of the flotation column and the hydrophilic material drops to the bottom. In the case of separating battery material, the graphite is typically made hydrophobic and will be pushed to the top with the flow of air bubbles, and the metal oxide is hydrophilic and will drop to the bottom, thus separating the two phases.
Unfortunately, the process is complicated by the very small particle size of the materials, which limits the impact of the bulk density, making it more difficult to sink, and because the materials contain a significant amount of polymer binder which makes the chemistry of the surfaces of the materials comprising the black mass behave in a similar manner. As a result, in practice the separation of the battery active materials by this method is typically very poor, with much of the cathode material behaving like the graphite anode material and floating to the top and much of the anode material behaving like the metal oxide cathode materials and sinking to the bottom.
Improvements have been made by trying to remove the binder prior to flotation separation by washing with various solvents at elevated temperature. However, this is difficult to do because of the low solubility of some of the binders and the high surface area of the battery active materials, and to date has not resulted in significant increases in the separation efficiency of these materials. More preferably, the removal of the binder can be easily accomplished later in the process by firing the materials in air once they have been purified. However, firing the black mass to remove the binder prior to separation has a negative impact on the subsequent performance of the active materials. Furthermore, the aqueous method of separation and the surfactants that are used to functionalize the materials can have a negative impact on the performance of the materials once they are recovered. Thus, it is desirable to have a method in which the recovered anode and cathode active materials can be separated prior to removal of the binder and in a medium that is inert to the sometimes highly reactive anode and cathode materials recovered from a Li-ion cell, and with high throughput to limit the time between material separation and regeneration of the materials for reuse in a new cell.